Laser Diode Readers: The Precision of Coherent Light in Barcode Scanning |
Executive Summary |
This article provides a comprehensive exploration of laser diode readers in barcode scanning applications, focusing on the unique advantages and design considerations that distinguish them from LED-based systems. We examine how the coherent, collimated beam of a laser enables long-range scanning, high resolution, and superior performance on challenging surfaces. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real patent disclosures from industry leaders including U.S. Philips Corporation, PSC Inc., Symbol Technologies, and other pioneering companies. We explore the dual-facet nature of laser diodes as both light sources and detectors, the automatic power control (APC) loop that maintains constant optical output, the integration of laser drive and signal processing on single chips, and the practical challenges of invisible beams and safety regulations. The article covers both the fundamental principles and the practical implementation details that make laser-based readers the preferred choice for demanding applications. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting laser diode readers for barcode reading applications. |
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Chapter 1: The Coherent Advantage |
The laser diode represents a fundamental shift in barcode reading technology. Unlike an LED, which emits light in a broad, divergent beam, a laser diode produces a coherent, collimated beam that can be focused to a very small spot over a long distance. This allows a laser reader to read barcodes from much greater distances than an LED-based system and to resolve finer details, making it suitable for high-density barcodes. |
The laser beam is generated by a semiconductor laser chip with two reflective end faces that form an optical cavity. Light bounces back and forth between these faces, building up intensity until a coherent beam is emitted from the front facet . The key property of this beam is its coherence: the light waves are all in phase, which allows them to be focused to a diffraction-limited spot. This is the source of the laser's superior resolution and range. |
The early adoption of laser diodes in barcode readers was driven by the desire for miniaturization. Gas lasers like helium-neon tubes were bulky and power-hungry. Semiconductor laser diodes offered a much smaller footprint and lower power consumption, but they had a critical drawback: they typically emitted invisible infrared light, making it difficult for the operator to aim the reader . This challenge drove innovations in aiming beam technology that are still used today. |
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Chapter 2: The Dual-Facet Laser Diode |
A key innovation in laser diode readers is the dual-facet nature of the laser chip. The laser chip has two end faces, both of which emit light. The front facet emits the main beam that is directed to the barcode. The rear facet emits a smaller beam in the opposite direction, which is used for monitoring and control . |
In a conventional laser diode package, the rear facet beam is directed onto a monitor photodiode that is part of a feedback loop. The monitor photodiode is mounted within the casing, rearwardly of the laser chip and radially offset from the optical axis . The photodiode 'watches' the rearwardly-directed beam and generates a current proportional to the laser's output power. This current is used by a current controller to adjust the forward current to the laser chip, maintaining a constant optical output . |
What makes the laser diode reader truly innovative is that the monitor photodiode can serve two functions simultaneously. The U.S. Philips patent describes an arrangement where the laser diode has two end faces: one coupled to an optical fiber for scanning the barcode, and the other coupled to a detector for reading the reflected light via optical feedback . The reflected light from the barcode is coupled back into the laser, causing variations in the laser's output power, which are detected by the monitor photodiode. This eliminates the need for a separate photodetector and directional coupler, simplifying the reader design. |
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Chapter 3: Automatic Power Control --- The Feedback Loop |
The automatic power control (APC) loop is essential for safe and reliable laser operation. Laser diodes are sensitive devices; their output power can vary with temperature, aging, and manufacturing tolerances. The APC loop maintains the optical output at a constant, safe level by adjusting the forward current. |
The APC loop works as follows: the monitor photodiode produces a photocurrent that is proportional to the laser output power. This photocurrent is fed back to a controller, which compares it to a reference and adjusts the laser drive current accordingly . If the laser output drops, the photocurrent drops, and the controller increases the drive current. If the output rises, the controller reduces the drive current. |
The Symbol Technologies patent describes the importance of this feedback system: 'the laser diode assembly 46 is a three terminal device that includes a laser diode 80 (LD) and a monitor photodiode 82 (PD). A small fraction of the output laser light is coupled into the monitor photodiode 82 within the enclosed assembly 46. This induces a photocurrent I_m in the monitor photodiode that is proportional to the laser output power. This photocurrent I_m is the negative feedback signal that is used to regulate the laser's output power' . |
In advanced implementations, the APC loop is under microprocessor control. The Symbol Technologies patent describes how 'the laser drive current and the laser output power can be monitored by the microprocessor, and the reader can be shut down in the event of a laser malfunction' . The microprocessor can also adjust the laser output power 'for increased performance,' such as when scanning at the ends of each scan line where the beam velocity is nearly zero. |
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Chapter 4: The Invisible Beam Challenge |
The most significant challenge with early laser diode readers was the invisibility of the beam. Most semiconductor lasers emit in the infrared spectrum, typically around 815 nanometers, which is invisible to the human eye. This made aiming the reader difficult, as the operator could not see where the beam was pointing . |
The invisibility problem was addressed by the development of visible laser diodes, such as those emitting at 670 nanometers. The patent from a barcode reader manufacturer describes using a high-power laser diode which emits a light beam having a wavelength of 670 nanometers and an average power output of 5 milliwatts. This beam is visible even in bright sunlight, allowing the operator to aim the apparatus accurately . |
The use of a visible laser also improves the reading performance. 'First, a stronger reflected light signal will be reflected from the barcode when a higher power laser beam is used which increases the usable signal received and thus requires less amplification and consequently reduces the risk of saturation of the amplification circuitry,' the patent explains . |
For applications where infrared lasers are still used, the marker beam technique was developed. An optical device described in a patent uses an invisible laser beam to scan the symbol while simultaneously illuminating the symbol with a visible marker beam that is coincident with the laser beam. This 'enables the operator to locate the laser beam with respect to the code symbols, such that the code symbols can be scanned with a high degree of assurance that the laser beam will be reflected from the code symbols' . |
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Chapter 5: High-Power Lasers and Glare Rejection |
The use of high-power laser diodes has a second major benefit: improved glare rejection. When scanning barcodes on reflective surfaces or in bright ambient light, the received signal can be swamped by glare, causing saturation of the amplification circuitry and loss of data . |
The patent from a barcode reader manufacturer explains the problem with a specific example: 'the circuitry designed to receive and process the reflected light signal saturates at a signal strength of 4 volts or more... The noise caused by glare ... includes an area 204 in which data is irretrievably lost due to saturation of the circuitry' . |
The solution is to use a high-power laser combined with a narrow band optical filter. The high-power laser increases the signal-to-noise ratio, 'making it possible to lower the gain of the pre-amplifier circuit used to process the received reflected light signals from the barcode' . The narrow band optical filter, tuned to the laser wavelength (e.g., 670 nanometers with a 20-nanometer bandwidth), filters out other wavelengths that would appear as noise . This combination reduces the DC level and glare by approximately 13 dB, preventing saturation and preserving the data. |
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Chapter 6: Laser Spot Size and Noise Rejection |
The spot size of the laser beam is a critical parameter for reading performance. A smaller spot can resolve finer details, but it is also more sensitive to surface texture noise, especially on materials like metal. A larger spot is less sensitive to texture noise but may miss narrow bars. |
The patent from a barcode reader manufacturer describes this trade-off: 'metal surfaces for metal-imprinted barcodes, as well as other surfaces, are a potential source of additional high frequency noise which is not screened out by the high-pass filter as that noise is within the band of that filter' . A small spot on a metal surface can pick up noise from the surface texture, resulting in erroneous decoding . |
The solution is to focus the laser to a spot size that is 'substantially the same size as the smallest feature of the barcode to be scanned' . In the preferred embodiment, the laser is focused such that a spot size of approximately 202 micrometers exists at a scanning distance of 215 millimeters, a spot size of approximately 226 micrometers at a scanning distance of approximately 280 millimeters, and a spot size of approximately 260 micrometers at a scanning distance of 155 millimeters . This larger spot size filters out the high-frequency surface noise while still resolving the barcode features. |
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Chapter 7: The Scanning Mechanism |
The laser beam must be scanned across the barcode to create the time-varying signal. This is typically done with a moving mirror. The scanning mechanism is a critical component that affects the scan speed, accuracy, and power consumption. |
A patent from PSC Inc. describes a barcode scanner where the laser beam is reflected from a polygon beam deflector (a rotating multi-faceted mirror) that sweeps the beam along a scan path . The deflector is provided with structural means, such as radial ribs, that interrupt a light beam, generating a control signal that gates the laser on and off. The arrangement is such that 'the laser source will be enabled to generate the beam only within the confines of the scan path' . This reduces the duty cycle of the laser, conserving power. |
The scanning mirror can be mounted on a printed circuit board that also carries the laser diode, optics, and signal processing circuitry. The patent from a barcode reader manufacturer describes a 'unitary structure insertable into position and fixable in said housing after this structure has been assembled' . This modular design simplifies manufacturing and assembly. |
In gun-shaped readers, the scanning is performed by an oscillating mirror coupled to a scanning drive motor. The user aims the reader at the barcode and pulls a trigger, activating the laser and the scanning motor . |
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Chapter 8: The Integrated Optical Tube |
A key design feature of many laser diode readers is the integrated optical tube that houses the laser, optics, and detector. The patent from a barcode reader manufacturer describes an arrangement where 'the laser diode package, positive focusing lens, aperture stop, and negative collecting lens can advantageously be mounted in an optical tube' . The tube can be mounted on an oscillatable motor for sweeping the beam, or fitted onto the end of a wand for manual reading. |
The integration of the optical components into a single assembly provides several advantages. It ensures precise alignment between the laser beam and the collection optics, improves the robustness of the scanner, and simplifies assembly. The optical tube can be mounted in a housing for a fixed installation or in a handheld device for portable use . |
The integration can be even more advanced, with the laser and optics supported directly on a printed circuit board. The patent from a barcode reader manufacturer describes a scanner where 'said support plate is a printed circuit board carrying... the laser diode, the said electric circuitry and also said optics for forming the beam emitted from the laser diode' . This level of integration reduces the number of parts and the overall size of the scanner. |
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Chapter 9: The Unitary Scan Engine |
The ultimate expression of integration is the unitary scan engine, where all components --- laser diode, optics, detector, and electronic circuitry --- are assembled into a single, self-contained unit. The patent from a barcode reader manufacturer describes such a 'unitary structure insertable into position and fixable in said housing after this structure has been assembled' . |
The unitary scan engine offers several benefits. It simplifies manufacturing by reducing the number of assembly steps. It improves reliability by minimizing the number of interconnections and adjustments. It enables the scanner to be used in a variety of housings and configurations. |
The Symbol Technologies patent describes a 'unique dual chip architecture' where an ASIC (Application Specific Integrated Circuit) and a microprocessor share the control function of a reading session. 'Together, these two chips share the control function of a reading session... The board, together with the laser, optics, detector, and motor, are supported on a common support which comprises the above-described compact scan engine' . The ASIC integrates the receiver, digitizer, laser drive, motor drive, and temperature sensor circuits, while the microprocessor handles the decoding and system control. |
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Chapter 10: The LASER Safety Considerations |
Laser safety is a paramount concern in the design of laser diode readers. The laser beam must be contained within the scanner housing and only emitted through the output window when the scanner is properly aimed and activated. The scanner must also comply with regulatory requirements for laser safety, such as the FDA/CDRH regulations in the United States. |
A patent from PSC Inc. describes a laser scanner where a light beam is interrupted by structural means to generate a control signal that gates the laser on and off . The control signal ensures that the laser is enabled only when the beam is within the scan path and not, for example, at the ends of the scan where the beam would be stationary and potentially hazardous. This reduces the duty cycle of the laser, conserving power and enhancing safety. |
The Symbol Technologies patent describes a reader with safety circuits for detecting when the laser output is not within prescribed limits. 'The laser drive circuit 26 includes a laser power regulator, which is a closed loop feedback system that maintains a constant optical output power... [and] safety circuits for detecting when the laser output is not within prescribed limits' . If the laser output exceeds a safe level, the safety circuits can shut down the laser. |
The patent also describes how 'the microprocessor can turn off the laser at the ends of each scan line (when the velocity of the laser beam spot is nearly zero), thereby enabling the scan engine to operate at a higher laser output power for increased performance' . This is an elegant solution: the laser is powered down at the points where the beam would be stationary and potentially hazardous, allowing the average output to be safely increased. |
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Chapter 11: The Practical Design of Laser Diode Scanners |
The practical design of a laser diode scanner involves integrating the laser diode, scanning mechanism, optics, and electronics into a compact, reliable package. The patent from a barcode reader manufacturer provides a detailed example of a hand-held scanner. |
The scanner is contained in a housing with a handle and a beam port at the front . The laser diode package, a positive lens, and an aperture stop are aligned to focus the beam. The laser beam is directed to a mirror that scans the beam across the barcode . |
The reflected light is collected by the same lens and directed to a photodetector. In the integrated optical tube design, the reflected light is collected by the negative collecting lens and focused onto the monitor photodiode . The monitor photodiode, which is already part of the laser diode package for power control, is used as the receiver. |
The collection and detection of the reflected light is a critical aspect of the design. The patent from a barcode reader manufacturer describes the use of an aperture stop to define a constant illumination of the photodetector over the depth of field. The aperture 'does not limit the amount of light gathered from a maximum point of said depth of focus while limiting the collection of light from a minimum point of said depth of focus to the photosensitive area of said detector' . This ensures consistent signal amplitude over a wide range of reading distances. |
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Chapter 12: Bar Code Scanner with an Integrated Laser Diode |
The concept of a fully integrated laser diode scanner was pioneered in the 1980s. The patent from a barcode reader manufacturer describes a bar code scanner 'which is incorporated in a cash register' or 'at least partially accommodated in a light pen,' using a semiconductor laser diode with a second end face coupled to a detector for 'detection via incoherent optical feedback by the semiconductor laser diode and the optical waveguide' . |
The laser's output variations, caused by feedback of the reflected light, result from a shift in the laser's L-I characteristic. When feedback occurs, the threshold current is lowered, and for a given input current, more power is output. The detector detects this power increase and generates an electrical signal . |
The U.S. Philips patent also notes that the laser should emit light with a wavelength greater than 1.3 micrometers, which is 'completely harmless to the human eye, because radiation having this wavelength is absorbed by water' . The preferred laser is an InGaAsP diode laser, emitting between 1.3 and 1.5 micrometers. This choice reflects the historical concern for eye safety, although modern barcode readers typically use shorter-wavelength visible lasers with safety measures in place. |
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Chapter 13: The ASIC and Microprocessor Architecture |
The modern laser diode reader relies on a combination of an ASIC for the analog and high-speed digital functions, and a microprocessor for the decoding and system control. The Symbol Technologies patent describes this architecture in detail. |
The ASIC integrates several key circuits: 'a receiver 22, a digitizer 24, a laser drive circuit 26, a motor drive circuit 28, a temperature sensor circuit 30, and a voltage regulator circuit 34' . The receiver includes amplifier stages, an active filter, and an automatic gain controller. The digitizer processes the analog signal with differentiating circuits, peak detectors, and comparators. The laser drive circuit energizes the laser, with electronic beam clipping for chopping the laser drive current, and safety circuits for detecting when the laser output is not within prescribed limits. The motor drive circuit actuates the oscillating mirror, using positive feedback through a mechanical filter and discrete time signal processing. |
Each of these circuits has an element that can be adjusted under control of the microprocessor: 'The adjustable element 32 includes one or more of a digital potentiometer, a programmable control register, or switches' . The microprocessor can adjust the receiver bandwidth, digitizer thresholds, laser output power, motor amplitude, margin thresholds, digitizer hysteresis, and other parameters. This allows the reader to be optimized for different symbologies, lighting conditions, and scanning applications. |
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Chapter 14: Power Conservation in Laser Scanners |
In a laser scanner, the laser diode is the biggest power consumer. A patent from PSC Inc. describes a control circuit that reduces the laser's duty cycle, conserving power and extending battery life. |
The circuit generates a control signal from structural means on the rotating polygon deflector. The structural means, such as radial ribs, interrupt a light beam, producing a signal that gates the laser on and off. 'The arrangement is such that the laser source will be enabled to generate the beam only within the confines of the scan path' . This reduces the duty cycle of the laser, as the laser is off when the beam is scanning outside the scan path. |
This is particularly important for handheld scanners, where the laser is the 'element of the system which consumes the most power.' Reducing the laser's duty cycle is 'a major factor defining the life of batteries and the length of time of reliable operation with a given battery set or battery charge, and before the battery will have to be replaced or recharged' . |
The Symbol Technologies patent describes another power-saving technique: 'the microprocessor can turn off the laser at the ends of each scan line (when the velocity of the laser beam spot is nearly zero)' . This not only saves power but also enables the scan engine to operate at a higher laser output power for increased performance, since the laser is only on when the beam is moving. |
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Chapter 15: The Multi-Laser Array Design |
A more recent innovation is the multi-laser array design, which improves scanning performance by using multiple laser diodes. A Chinese patent describes a laser barcode reading device with multiple laser diodes arranged in a ring around a rotating polygon mirror . |
In this design, 'at least two laser diodes, for example including four laser diodes 8a, 8b, 8c, 8d, are arranged at equal intervals around the outer periphery of the ring-shaped reflector group' . A timing control device sequentially drives the multiple laser diodes to generate laser light, with only one laser diode being driven at a time. The rotation of the polygon mirror is detected by an optocoupler, which provides the timing signal . |
The multi-laser design can produce a full-angle scanning pattern, improving the reading performance for barcodes in any orientation. The lasers are directed onto the polygon mirror, which reflects them onto the ring-shaped reflector group, and out through the scanning window. This creates a complex scanning pattern that covers a wide area, increasing the chances of a successful read. |
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Chapter 16: Summary --- Laser Diode Readers in Perspective |
Laser diode readers represent a significant advancement in barcode scanning technology, offering long reading distances, high resolution, and superior performance on challenging surfaces. The coherent, collimated beam of a laser allows it to be focused to a small spot over a long range, making it ideal for industrial and high-density applications. The use of a high-power visible laser also provides excellent glare rejection and allows the reader to be aimed accurately. |
We have examined how different companies and technologies have approached the challenges of laser diode reader design: |
U.S. Philips Corporation developed an early bar code reader using a semiconductor laser diode with a second end face coupled to a detector, eliminating the need for a separate photodetector and directional coupler. The laser's output variations, caused by feedback, are detected by the monitor photodiode . |
PSC Inc. developed a control circuit for power conservation in laser scanners, using structural means on a rotating polygon deflector to generate a control signal that gates the laser on and off. The company also developed an optical device using a visible marker beam coincident with an invisible laser beam . |
A barcode reader manufacturer developed a high-power visible laser scanner with a narrow band optical filter, addressing the glare problem and enabling reading on reflective surfaces. The patent also describes the use of a focused spot size to reject surface noise . |
A barcode reader manufacturer developed a unitary scan engine with an ASIC and a microprocessor sharing the control function. The ASIC integrates the receiver, digitizer, laser drive, motor drive, and temperature sensor circuits . |
A Chinese patent describes a multi-laser array design with multiple laser diodes arranged in a ring, producing a full-angle scanning pattern . |
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The key lessons from our exploration are: |
Laser diodes provide superior range and resolution. Their coherent, collimated beam can be focused to a small spot over a long distance, enabling reading of high-density barcodes at long range. |
The dual-facet laser diode simplifies the reader design. The rear facet beam can be used for both power control and signal detection, eliminating the need for a separate photodetector. |
Automatic power control is essential. The APC loop maintains a constant optical output, ensuring safe and reliable operation. The microprocessor can monitor and adjust the laser power. |
Visible lasers solve the aiming problem. A visible laser allows the operator to see where the beam is pointing, improving the reading speed and accuracy. |
High-power lasers and optical filters reject glare. The combination of a high-power laser and a narrow band optical filter increases the signal-to-noise ratio and prevents saturation from glare. |
Laser spot size must be chosen carefully. A smaller spot resolves finer details but is more sensitive to surface texture noise. A larger spot rejects noise but may miss narrow bars. |
The scanning mechanism must be precise. The laser beam must be scanned accurately and consistently. The scanning mirror can be mounted on a printed circuit board for integration. |
Power conservation is critical in handheld scanners. The laser is the biggest power consumer, so techniques like reduced duty cycle and end-of-scan turn-off are essential for battery life. |
Integration is the trend. The laser diode, optics, detector, and electronic circuitry are being integrated into single, self-contained scan engines, reducing size and cost. |
In the end, laser diode readers are a testament to the power of coherent light. They bring the precision of a laser to the practical task of reading barcodes, extending range, improving resolution, and unlocking new applications. The art of laser diode reader design lies in the careful balance of power, optics, and electronics, creating a scanner that is safe, reliable, and high-performance in the real world. |
cs@easiersoft.com